Electronic weighing system for batching operations

ABSTRACT

An electronic weighing system for batching operations which includes a load cell for weighing a principal container which receives a number of ingredients from individual ingredient sources. The load cell generates an output signal in proportion to the instantaneous weight received in the container, and this signal is utilized to turn &#39;&#39;&#39;&#39;off&#39;&#39;&#39;&#39; one ingredient channel and to actuate a second channel. The output from the load cell is compared with an input signal associated with each channel which determines the loading which is to be added to the container from each ingredient channel. The input load signal for each channel is derived respectively from a number of potentiometers which are coupled in series in such a way as to provide the correct input comparison signal to produce the proper load output for each ingredient channel. This is accomplished by successively switching into series arrangement each new channel after the loading requirements of the previous channel have been satisfied. In this way, the input signal established at each channel in combination with the input signals of previously operated channels always exceeds the load cell output signal by a magnitude just sufficient to be satisfied by the predetermined ingredient loading demands of the channel in operation.

Jones et al.

ELECTRONIC WEIGHING SYSTEM FOR BATCHING OPERATIONS [151 3,655,002 [451Apr. 11, 1972 PrimaryExaminer-Rkhard B. Wilkinson AssistantExaminer-George H. Miller, Jr.

[ Inventors: l tvi Park; Attom --l-lill, Sherman, Meroni, Gross&Sim sonHamilton, Arlington Heights, both of m. p [73] Assignee: The Dole ValveCompany, Morton Grove, [57] ABSTRACT An electronic weighing system forbatching operations which [22] Filed: Sept 20, 1968 includes a load cellfor weighing a principal container which receives a number ofingredients from individual ingredient PP 761,087 sources. The load cellgenerates an output signal in proportion to the instantaneous weightreceived in the container, and this signal is utilized to turn off oneingredient channel and to 25 actuate a second channel. The output fromthe load cell is [58] Field 210 6 compared with an input signalassociated with each channel which determines the loading which is to beadded to the container from each ingredient channel. The input loadsignal for [56] References Cited each channel is derived respectivelyfrom a number of poten- UNITED STATES PATENTS tiometers which arecoupled in series in such a way as to provide the correct inputcomparison signal to produce the 368,491 1/1959 Thomson at "177/70proper load output for each ingredient channel. This is accom- ,938,70l5/1960 Thorsson et al. l 77/70 plished by successively switching intoseries arrangement each 9 10/1963 Williams t "177/70 new channel afterthe loading requirements of the previous t 3/1964 set 177/70 channelhave been satisfied. In this way, the input signal ,l73,504 3/l965Thorsson et al. .....l77/7 established at each channel in combinationwith the input 173,505 3/1965 Thomson -177/70 signals of previouslyoperated channels always exceeds the ,203,591 8/ 1965 Daulton et al...l77/70 x load cell output signal by a magnitude just sufficient to be317,927 I l/ 1965 Bale at 177/70 x satisfied by the predeterminedingredient loading demands of ,22 l ,828 12/1965 Kohler 177/70 X thechannel in operation. 263,761 8/1966 Boadle et al ..l77/70 ,434,5563/1969 Bale, Jr ..l77/70 12 Claims,3Drawing Figures 1 Q arc/114702 I /Z8wrsezooz 2am I l as l .49 2a 4? l l Man 4 arm /r52 arm Ala #41 4 0.? lf- #0? l I l l l -e l l I l i 2a l I I l i -J l I 251), EEM) l ,ezzm

[/21 K6 Fill fill E PATENTEDAPR 1 1 I972 SHEET 1 0F 3 BACKGROUND OF THEINVENTION 1. Field of the Invention The field of art to which thisinvention pertains is an electronic system for continuously monitoringthe weight of ingredients being fed into a common container and forcontrolling a series of operational stations in response to the changingloading of the container to assure proper feeding proportions of therespective ingredient input channels.

2. Summary It is an important feature of the present invention toprovide an improved electronic weighing system for batching operatrons.

It is also a feature of the present invention to provide a highlyefficient control circuit to regulate the operation of a series ofingredient channels for supplying predetermined quantities of specifiedingredients to a common container.

It is an important object of the present invention to provide animproved control circuit utilizing the output of a load cell as a meansof triggering a number of operational stations for supplying preselectedquantities of specified ingredients to a container, the weight of whichis being sensed by the indicated load cell.

It is another object of the present invention to provide a controlcircuit having a series of input selection units which are connected inseries with the output of a load cell and which are selectivelyconnected into series with each other in response to a null conditionbeing received at a previously connected load selection unit.

It is also an object of the present invention to provide a controlcircuit which utilizes the continuous output from a load cell in serieswith a number of serially connected potentiometers wherein each of thepotentiometers has an output signal equal to the combined signals oftheload cell output and a reference signal applied to the potentiometer.

It is a further object of the present invention to provide an electroniccontrol circuit including a number of potentiometers having a fixedreference voltage applied thereacross and having the output from a loadcell coupled to one terminal of the potentiometer such that the outputfrom the load cell adds algebraically to the utilized portion of thereference signal applied to the potentiometer for controlling theoperation of an ingredient feeding station and for switching anadditional potentiometer into series connection with the system toactuate the same and deactuate the previous ingredient feeding channel.

It is an additional object of the present invention to provide aninterlock circuitry for an electronic control network as described abovewherein a holding circuit maintains each channel in an inoperative stateafter once having completed its cycle until all of the ingredients ofthe container being sensed are removed and the weight being sustained bythe load cell is relieved.

It is another object of the present invention to provide an amplifierfor each of the respective ingredient channels which amplifiers have ahigh input impedance for drawing substantially zero current from saidpotentiometers.

These and other objects, features and advantages of the presentinvention will be understood in greater detail from the followingdescription and the associated drawings wherein reference numerals areutilized in designating an illustrative embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram illustratingthe functional relationship of the various components comprising theelectronic control circuitry of the present invention;

FIG. 2 is a schematic of the channel transfer circuitry showing theindividual channel load selection units or potentiometers and theassociated relays for connecting the same in series with each other andwith the load cell output, and

FIG. 3 is a schematic of the interlock circuitry of the presentinvention which is utilized to maintain each respective channel in aninoperative state, after once having been cycled, until the principalingredient receiving container is emptied.

DESCRIPTION OF THE PREFERRED EMBODIMENT The weighing system of thepresent invention may consist, for example, of a hopper which issupported by a load cell or series of load cells which, in turn,generates an output signal indicative of the load received by thehopper. A number of ingredient feeding stations may be oriented directlyabove the hopper to supply material being mixed to the hopper inaccordance with a predetermined composition.

Essentially, the control circuit of the present invention turns on" afirst ingredient feeding station and allows material to be fed to thehopper until the weight as registered by the load cell reaches thepreselected weight of material for that par ticular ingredient. At thattime, the first feeding station is turned off, and a second feedingstation is turned on" for introducing a second ingredient into thehopper. When the hopper reaches the desired weight in accordance withthe load already supplied by the first ingredient feeding station andthe load required by the second station, the second feeding station isturned off and subsequent feeding stations are initiated.

The circuit of the present invention provides an efficient and effectivemeans for accomplishing this function by providing a series ofpotentiometers which effectively introduce an input control signal toeach ingredient feeding station which signal must be counterbalanced bythe output of the load cell to turn off" that station and to transferoperation to a sub sequent channel.

More specifically, when the voltage input or reference signalcorresponding to the first channel is balanced or nulled by the outputof the load cell, indicating that a full measure of the first ingredienthas been supplied to the hopper, a second potentiometer is connected inseries with the first potentiometer, thereby adding a second referencesignal in series with the reference signal associated with the firstchannel. The combination of the first and second reference signals thenexceeds the output of the load cell, and accordingly, the second feedingstation is initiated. This process is then repeated for the number offeeding stations which are required to develop the specified compositionof materials.

After the last feeding station has fulfilled its demand requirements,the hopper is ready to be emptied. The emptying of the hopper, ofcourse, reduces the output signal of the load cell and would tend tore-engage each of the ingredient feeding stations. To preventre-engagement during emptying of the hopper, a zero interlock circuit isprovided which holds each of the channels in a deenergized state untilall of the contents of the hopper have been removed. At this time, theentire cycle may be repeated.

Referring specifically to the block diagram shown in FIG. 1, a load cell10 is shown to be supplied by an oscillator 11 at a frequency, forinstance, of approximately 2,500 I-lz. A balance potentiometer indicatedgenerally at 12 is provided to null out the output signal as developedat circuit point 13 under noload conditions. The balance control 12 maybalance out the weight of the hopper, for instance, and other associatedapparatus permitting the load cell assembly to generate a signal whichis indicative of the weight of the ingredients alone.

The output signal from the load cell and balance network is coupled fromthe point 13 to an amplifier l4 and from the amplifier 14 to a circuitjunction point 15. The amplified load indicative signal is then coupledto a first channel 16. The channel 16 controls a valve 17 through arelay 18 and determines the weight of ingredients added to the hopper.

When the load cell 10 senses that the full measure of ingredient fromthe first channel has been fed into the hopper, the valve 17 is closed,and the relay 18 couples a second channel 19 into series with the firstchannel 16. In a similar manner, the second channel is provided with arelay 20 and a control valve 21. When the desired quantity of ingredientfrom the second channel is received into the hopper, a null condition isdeveloped in the second channel 19 for actuating the relay 20 andclosing the second ingredient valve 21. At the same time, the relay 20couples a third ingredient channel 22 into series engagement withchannel 1 and channel 2.

The third channel is provided with a relay 23 and a valve 24 andoperates in a manner similar to the operation of channels 1 and 2. Theprocess may be repeated for a number of channels corresponding to thenumber of ingredients required for the given mixture.

Each of the relays 18, 20 and 23 include a holding circuit formaintaining the relay in an energized condition after once having beenenergized at the time of the turning off of the respective channel. Thisholding circuit is released by means of an interlock circuit 25 which iscoupled to a control relay 26. The relay 26 is in turn coupled to eachof the relays 18, 20 and 23 such that the opening of the relay 26releases the respective holding circuits thereby placing the channeltransfer circuits in a ready position for a subsequent cycle.

Regarding the balancing network 12 for balancing out system weight, amovable tap 27 is provided to adjust the balancing signal applied to thecircuit point 13. The tap 27 may be adjusted until a zero output signalis provided at the circuit junction point 15 as indicated by a meter 28.

Referring to FIG. 2, the output from the amplifier 14 is indicatedgenerally at terminal 29. This output signal is then fed through acircuit line 30 and through a contact 31 to a terminal 32 at apotentiometer 33.

The potentiometer 33 includes a resistor 34 and a movable tap 35. Atransformer 36 couples a reference signal from a primary winding 37 to asecondary winding 38. The output of the secondary winding 38 is applieddirectly across the resistor 34, and a portion of this reference signalis developed between the terminal 32 and the movable tap 35.

The potentiometer 33 corresponds to the load selection unit forchannel 1. Similar potentiometers 62 and 620 are provided for the loadselection units of channels 2 and 3, respectively. Each of thepotentiometers 62 and 62a are provided with input transformers 41 and 42respectively. These transformers have primary windings 43 and 44 andsecondary windings 45 and 46 for coupling the indicated reference signalacross the respective resistors 47 and 48. Each of the primary windings37, 43 and 44 are connected in parallel and a reference signal isapplied thereto at a pair of terminals 49 and 50. The signal which isdeveloped across the secondary windings 38, 45 and 46, respectively,consist of an AC signal equal in frequency to the frequency applied tothe load cell'and 180 out of phase therewith.

Referring specifically to the first channel, the movable tap whichsenses the combined signals developed from the load cell at the terminal29 and from the reference signal, is coupled directly to an amplifier 51which in turn'controls a relay 52, the relay 52 being coupledto groundas at 53.

In operation, initially, the load cell produces zero output signal whenthe hopper or receiving container is empty. The reference signal asapplied between the movable tap 35 and the terminal 32, then developsasignal for turning on the amplifier 51 and turning the relay 52 off. Atthis time; the ingredient associated with the first channel begins toflow into the hopper, thereby adding weight to the load cell. Thegradually increasing weight in the hopper generates a signal at theterminal 29 which opposes the signal across the resistive portion 54between the movable tap 35 and the terminal 32. When a null condition isreached, the required load from channel 1 has been received within thehopper and means must be provided to switch operation to a succeedingchannel. It is apparent, then that the quantity of material beingdelivered from channel 1 can be adjusted by adjusting the movable tap 35thereby increasing or decreasing the voltage drop across the resistiveportion 54 of the resistor 34.

Initially, a relay contact 55 which is coupled from'a circuit point 56to a circuit point 57 is normally open. However, the

turning on of the relay 52 closes the relay contact 55 and couples theterminal 57 directly to the movable tap 35. The terminal 57 is coupledthrough a normally closed switch 58 and a circuit line 59 to a circuitjunction point 60 at the potentiometer 47 which corresponds to thecircuit junction point 32 associated with the potentiometer 54. A secondrelay contact 61 which is normally closed is opened by the actuation ofthe relay 52.

As in the case of channel 1, the resistor 47 has a resistive portion 62coupled between a movable tap 63 and the circuit junction point 60. Themovable tap may be adjusted to develop the desired signal across theportion 62 which is utilized to control the quantity of ingredient whichis fed from channel 2 to the receiving hopper.

At the time of the closing of the relay contact 55, which as is shown,connects the potentiometer 47 in series with the movable tap 35 and withthe resistive portion 54, the voltage across the portion 54 was exactlycounterbalanced by the voltage developed at the output 29 of the loadcell and amplifier combination. Due to the added input voltageintroduced by the resistive portion 62, an amplifier 64 associated withthe potentiometer 47 and a relay 65 will continue to function and addingredients to the receiving hopper until the output of the load cellattains a new level as reflected by the combined voltages of theresistive portions 54 and 62.

When the supply of ingredients from the channel 2 satisfies therequirements of the potentiometer setting associated therewith, a nullcondition will be attained at the movable tap 63, thereby turning offthe amplifier 64 and turning on the relay contact 65, and hence, turningon" a relay contact 66 and connecting the movable tap 63 through aswitch 67 to a terminal point 68 associated with the potentiometer 62a.Similar to the relay contact 61, the relay contact 69 is opened, therebyallowing a series connection between the movable tap 63 and a movabletap 70 of the potentiometer 62a. The potentiometer 62a is provided withrelay contacts 71 and 72 which correspond to relay contacts 66 and 69,respectively, of potentiometer 47 for coupling the signal as developedat the movable tap 70 to subsequent channels. The movable tap 70 iscoupled to an amplifier 73 and to a relay 74 which corresponds to likeparts in the preceding channels.

The potentiometer 62a is also provided with a switch 75 similar to theswitches 58 and 67. These switches allow any one channel or combinationof channels to be removed from the sequence of operation. This isaccomplished simply by moving the respective switches from theirnormally closed position as shown to a position contacting the opposingterminal, such as the terminal 31 associated with the switch 58.

Each of the amplifiers 78, 51, 64 and 73 have a high input impedance.This high input impedance means that the respective amplifiers will drawsubstantially zero current. In this way there will be substantially noIR drop across the resistors 48, 47, 34, 76 and 77. Hence, theamplifiers will be responsive only to the voltage developed at theoutput of the load cells, thereby eliminating error due to the presenceof current in the indicated potentiometers and amplifiers. In thepreferred embodiment herein this high input of impedance is achieved bythe MOS-FET transistor which is well known in the art to have a highinput impedance;

The interlock feature illustrated at reference numeral 25 in FIG. 1 isshown schematically in FIG, 2 as including a pair of resistors 76 and 77coupled across a secondary 78 of a transformer 79. The primary of thetransformer 79 is coupled in' parallel with each of the primaries 37, 43and 44, and accordingly, the reference signal as applied at the.terminals 49 and 50 is applied across the resistors 76 and 77. Only asmall portion of the reference voltage is developed across the resistor77 which determines the on" point for the interlock. After the interlockis on, relay contact 80 closes, connecting amplifier 78" directly tocircuit point 29, Amp1ifier'78 and relay 79 and its contact'80-are heldin an on condition until substantially all'of the weight is removed fromthe receiving hopper.

The relay contact 82 of the relay 79 is shown in FIG. 3 along with abias input terminal 82a. The circuit in FIG. 3 shows a series of holdingcircuits associated with each channel, and shows the means for releasingthese holding circuits as consisting essentially of the relay contact 82which when opened removes a positive bias otherwise applied at theterminal 820. A main switch 83 is provided in the circuit to connect ordisconnect the interlock circuit from the network as desired.

Referring to channel 1, once the relay contacts 55 are closed, a signalis developed at the circuit junction point 84 intermediate a pair ofresistors 85 and 86 for biasing on a transistor 87 which in turn biaseson" a second transistor 88 thereby maintaining the energization of therelay 52. Accordingly, once the contacts 55 are closed, the holdingcircuit maintains the relay in a closed condition until the bias isremoved from the transistors 87 and 88 by the opening of the relaycontacts 82. As explained, these contacts are opened in response to thedeenergization of the relay 79 as shown in FIG. 2

In a similar manner, the relay 65 is maintained in an on condition oncethe contacts 66 have been closed due to a positive bias which issupplied to the circuit junction point 89 through the resistors 90 and91. Accordingly, the transistors 92 and 93 are maintained in an on"condition thereby holding on the relay 65.

Channel 3 is provided with corresponding resistors 94 and 95 andcorresponding transistors 96 and 97 to hold on the relay 74 once thecontacts 71 have been closed. It is apparent then that the respectiveholding circuits for each of the channels together with the relaycontact 82 provides a means for preventing the actuation of any channel,after that channel has once been actuated and prior to the emptying ofthe principal receiving hopper.

It will be apparent to those skilled in the art that variousmodifications and combinations of the features of our invention may bereadily achieved, but we desire to claim all such modifications andcombinations as properly come within the scope and spirit of ourcontribution as set forth herein.

We claim as our invention:

1. An electronic weighing system for batching operations comprising:

a load cell and means for generating an output signal therefrom,

container means being weighed by said load cell, a series of feedstation means for feeding material into said container means,

a plurality of potentiometers including an impedance element having aconstant voltage source applied thereacross and a movabletap associatedwith said impedance element,

a trigger circuit means associated with each of said potentiometers andbeing coupled to an associated feed station and including a high inputimpedance amplifier,

the output of said load cell being coupled to one terminal of saidimpedance elements,

said trigger circuit being responsive to the combined signals of saidload cell and said impedance element at said movable tap for activatingand deactivating said associated feed station, and

means for coupling a second one of said impedance elements in serieswith a portion of said first impedance element between said movable tapand said one terminal in response to the deactivation of said feedstation associated with said one impedance element, said means forcoupling including a relay connected to and triggered by said high inputimpedance amplifier and having contacts serially interposed between saidone impedance element and said second impedance element.

2. An electronic weighing system for batching operations comprising:

a load cell and means for generating an output signal therefrom,

container means being weighed by said load cell a series of feed stationmeans for feeding material into said container means,

a plurality of potentiometers including an impedance element having aconstant voltage source applied thereacross and a movable tap associatedwith said impedance element,

a plurality of trigger circuit means each associated with a separate oneof said potentiometers and being coupled to an associated feed station,the output of said load cell being coupled to one terminal of one ofsaid impedance elements,

the trigger circuit associated with said one impedance element beingresponsive to the combined signals of said load cell and said oneimpedance element at said movable tap for activating and deactivatingsaid associated feed station, and

means for coupling a second one of said impedance elements in serieswith the portion of said first element between said movable tap and saidone terminal in response to the deactivation of said feed stationassociated with said one impedance element.

3. A circuit in accordance with claim 2 wherein interlock means areprovided and coupled to and operated by a constant voltage source forholding said coupling means closed until said load cell is substantiallyunloaded.

4. A circuit in accordance with claim 3 wherein said interlock meanscomprises a reference potential source in parallel with saidpotentiometers and in series with said load cell output signal, meansfor holding said coupling means closed and trigger means responsive toboth said reference potential and said load cell output signal toactuate said holding means.

5. A control circuit comprising:

load sensitive means for generating an output signal indicative of theload received thereon,

a plurality of load selection units, each of said load selection unitsbeing connected in series with each other,

means for applying the output signal from said load sensitive means toone of said load selection units, means for coupling said output signalto another one of said load selection units through said one loadselection unit, and

a plurality of operational means coupled to individual ones of said loadselection units, each of said operational means having a cycle ofoperation and being responsive to its associated load selection unit toregulate the load received at said load sensitive means, means fordisconnecting said other one of said load selection units from said oneunit during the cycle of operation of said one unit and for connectingsaid other unit to said one unit subsequent to the cycle of operation ofsaid one unit.

6. A control circuit in accordance with claim 5 wherein each of saidload selection units comprises a potentiometer and means for applying areference signal across the same,

said potentiometer being adjustable to selectively establish anoperation voltage point for the associated operational means.

7. A control circuit in accordance with claim 6 wherein each of saidpotentiometers has a reference point, means for applying the output ofsaid load sensitive means to said reference point and means fordeveloping a portion of said reference signal between the point ofcoupling of said operational means and said reference point whichportion is out of phase with the output of said load sensitive means.

8. A control circuit in accordance with claim 7 wherein the operationalmeans associated with said one load selection means is normallyoperative until the output of said load sensitive means equals saidportion of said reference signal.

9. A control circuit comprising:

a load cell and means for developing an output signal in response to theloading of said cell,

a plurality of voltage selected input signal means, each signal havingan instantaneous polarity opposed to the polarity of the output of saidload cell,

input, second input and load cell output signal to control the furtherloading of said load cell, and means for disengaging said secondoperational means when a second predetermined loading is attained onsaid load cell.

10. A control circuit in accordance with claim 9 wherein said voltageselected input signal means comprises a potentiometer, a constant signalsource being applied across said potentiometer and a movable tap thereonfor determining the magnitude of the selected input signal.

11. A control circuit in accordance with claim 10 wherein the output ofsaid load cell is coupled to one terminal of one of said potentiometersand wherein the signal developed between the movable tap thereof andsaid one terminal combines with the output signal from said load cell todevelop a trigger signal, and wherein said trigger signal is applied tothe input of said first operational means.

12. A control circuit in accordance with claim 11 wherein said means forcoupling said second voltage selected input signal into series relationwith said one input signal comprises a normally open switching meanscoupled between said movable tap of said one potentiometer and oneterminal of another of said potentiometers and wherein means areprovided to close said switching means in response to the deactuation ofsaid first operational means.

1. An electronic weighing system for batching operations comprising: aload cell and means for generating an output signal therefrom, containermeans being weighed by said load cell, a series of feed station meansfor feeding material into said container means, a plurality ofpotentiometers including an impedance element having a constant voltagesource applied thereacross and a movable tap associated with saidimpedance element, a trigger circuit means associated with each of saidpotentiometers and being coupled to an associated feed station andincluding a high input impedance amplifier, the output of said load cellbeing coupled to one terminal of said impedance elements, said triggercircuit being responsive to the combined signals of said load cell andsaid impedance element at said movable tap for activating anddeactivating said associated feed station, and means for coupling asecond one of said impedance elements in series with a portion of saidfirst impedance element between said movable tap and said one terminalin response to the deactivation of said feed station associated withsaid one impedance element, said means for coupling including a relayconnected to and triggered by said high input impedance amplifier andhaving contacts serially interposed between said one impedance elementand said second impedance element.
 2. An electronic weighing system forbatching operations comprising: a load cell and means for generating anoutput signal therefrom, container means being weighed by said load cella series of feed station means for feeding material into said containermeans, a plurality of potentiometers including an impedance elementhaving a constant voltage source applied thereacross and a movable tapassociated with said impedance element, a plurality of trigger circuitmeans each associated with a separate one of said potentiometers andbeing coupled to an associated feed station, the output of said loadcell being coupled to one terminal of one of said impedance elements,the trigger circuit associated with said one impedance element beingresponsive to the combined signals of said load cell and said oneimpedance element at said movable tap for activating and deactivatingsaid associated feed station, and means for coupling a second one ofsaid impedance elements in series with the portion of said first elementbetween said movable tap and said one terminal in response to thedeactivation of said feed station associated with said one impedanceelement.
 3. A circuit in accordance with claim 2 wherein interlock meansare provided and coupled to and operated bY a constant voltage sourcefor holding said coupling means closed until said load cell issubstantially unloaded.
 4. A circuit in accordance with claim 3 whereinsaid interlock means comprises a reference potential source in parallelwith said potentiometers and in series with said load cell outputsignal, means for holding said coupling means closed and trigger meansresponsive to both said reference potential and said load cell outputsignal to actuate said holding means.
 5. A control circuit comprising:load sensitive means for generating an output signal indicative of theload received thereon, a plurality of load selection units, each of saidload selection units being connected in series with each other, meansfor applying the output signal from said load sensitive means to one ofsaid load selection units, means for coupling said output signal toanother one of said load selection units through said one load selectionunit, and a plurality of operational means coupled to individual ones ofsaid load selection units, each of said operational means having a cycleof operation and being responsive to its associated load selection unitto regulate the load received at said load sensitive means, means fordisconnecting said other one of said load selection units from said oneunit during the cycle of operation of said one unit and for connectingsaid other unit to said one unit subsequent to the cycle of operation ofsaid one unit.
 6. A control circuit in accordance with claim 5 whereineach of said load selection units comprises a potentiometer and meansfor applying a reference signal across the same, said potentiometerbeing adjustable to selectively establish an operation voltage point forthe associated operational means.
 7. A control circuit in accordancewith claim 6 wherein each of said potentiometers has a reference point,means for applying the output of said load sensitive means to saidreference point and means for developing a portion of said referencesignal between the point of coupling of said operational means and saidreference point which portion is 180* out of phase with the output ofsaid load sensitive means.
 8. A control circuit in accordance with claim7 wherein the operational means associated with said one load selectionmeans is normally operative until the output of said load sensitivemeans equals said portion of said reference signal.
 9. A control circuitcomprising: a load cell and means for developing an output signal inresponse to the loading of said cell, a plurality of voltage selectedinput signal means, each signal having an instantaneous polarity opposedto the polarity of the output of said load cell, means for connectingone of said voltage selected input signals in series with said load celloutput, first operational means responsive to the combined one input andload cell output signal to control the loading of said load cell, saidfirst operational means having a high input impedance, means fordisengaging said first operational means when a first predeterminedloading is attained on said load cell and for coupling a second voltageselected input signal into series relation with said one input signal,second operational means responsive to the combined one input, secondinput and load cell output signal to control the further loading of saidload cell, and means for disengaging said second operational means whena second predetermined loading is attained on said load cell.
 10. Acontrol circuit in accordance with claim 9 wherein said voltage selectedinput signal means comprises a potentiometer, a constant signal sourcebeing applied across said potentiometer and a movable tap thereon fordetermining the magnitude of the selected input signal.
 11. A controlcircuit in accordance with claim 10 wherein the output of said load cellis coupled to one terminal of one of said potentiometers and wherein thesignal developed between the movable tap thereof and said one terminalcombines with the output signal from said load cell to develop a triggersignal, and wherein said trigger signal is applied to the input of saidfirst operational means.
 12. A control circuit in accordance with claim11 wherein said means for coupling said second voltage selected inputsignal into series relation with said one input signal comprises anormally open switching means coupled between said movable tap of saidone potentiometer and one terminal of another of said potentiometers andwherein means are provided to close said switching means in response tothe deactuation of said first operational means.